1. Field of the Invention
This invention relates generally to an orthopedic implant assembly for holding and compressing together, adjacent bones, or bone portions. More particularly, this invention relates to a novel implant assembly for dynamically compressing adjacent bones together, wherein the dynamic compression is provided by polyaxial screws which may be locked to the plate.
2. Description of the Prior Art
Human bones are constructed of a hard external shell, referred to as cortical bone, and an interior which is less dense and is referred to as cancellous bone. Although bones are especially resilient and structurally remarkable with respect to the long term cycling of loading, which they are ideally constructed to support, a variety of trauma, as well as disease may cause bones to fracture. Bone fractures may be classified according to their severity, ranging from minor stress fractures to fully displaced fractures. Fractures are classified as "closed or "open", "open" describing those fractures wherein the skin is open and the fracture is exposed to the external environment.
Stress fractures, and other more mild bone breakage, occur when the bone develops a defect in its integrity, but otherwise is not structurally compromised in a gross manner. In healthy individuals, wherein the breakage is not causally related to a disease or other such pathology, stress fractures are often caused by excessive cyclical stress loading which exceeds the healing rate of the bone tissue. Such minor breaks, if not continuously aggravated, are generally treated by reducing the loading thereon for a short to intermediate period (a number of weeks). In some cases it is necessary to apply external immobilization to the bone or adjacent joints, in either a brace or a cast.
More serious fractures, including those in which the bone has a large structural defect, through those in which bone continuity is totally disrupted by a complete displacement of segments thereof, more drastic measures must be applied to correct the damage, and permit a "callus" to form around the bone to allow union of the fracture. In many cases it is sufficient to "set" the bone by initially placing the surfaces of the bone segments which need to be healed in contact with one another, and applying rigid external immobilization to protect the limb which has sustained the breakage. External casts and braces are utilized to provide stabilization of the bone, as well as to eliminate excessive stress loading during the healing process. Stress loadings of a light to moderate level are conducive to bone growth, however, if the stresses during healing are excessive, the fracture may not heal, and will alternatively go on to form a "non-union"
In more severe instances, however, the segments of the bone which is broken, and the manner in which the break manifests itself, makes the reduction of the fracture and the maintenance of the reduction difficult. In some cases the bone segments have been displaced to the extent that invasive surgical procedures must be carried out in order to openly reduce the bone segments together. In others, the specific bone which is broken (such as the femur or the hip) is one for which the time involved with total healing exceeds what may practically be externally immobilized for the duration. In still others, the ability of a cast to sufficiently relieve the loadings associated with a prudently constrained lifestyle is insufficient to permit correct healing. In these cases, and others, it is generally accepted that open reduction and internal fixation be provided, generally utilizing metal devices. Devices used for open reduction and internal fixation include screws, rods, and plates.
A very successful screw plate assembly, which has gained world-wide acceptance in the field of orthopedic surgery for the open reduction and internal fixation of long bone fracture is a dynamic compression plate (DCP). In its most widely used form, the DCP is an elongate planar metal plate having a plurality of sequentially spaced elongated holes, the holes being coaxial and extending along the axis of the plate. Each hole, which extends fully through the plate is defined by a pair of vertically oriented parallel sidewalls. The top of the parallel sidewalls, near the top surface of the plate, includes an additional recessed lip on which the annularly extending portion of the head of a screw rests once inserted into the hole. The top of the lip is, therefore, lower than the top surface of the plate. The elongate profile of the lip defines a downward slope with respect to the top surface of the plate, wherein the distance between the top of the lip surface and the top surface of the plate increases along the elongate axis of the hole. The downward slope of the lip of each hole is oriented toward the middle of the plate.
In use, the plate is positioned next to the exposed surface of the broken, and potentially displaced, bone such that one portion, an upper portion of the plate is adjacent one of the segments, and the holes of the other, lower portion, are adjacent to the other bone segment. First and second screws are inserted through the plate into opposing segments of the broken bone, at respective points along the axial extent of the elongate holes. It is preferred that the respective insertion points be at least some distance from the end of the hole having the lip at its greatest depth, herein referred to as the proximal end. In general, the screws are generally inserted at the uppermost end of the elongate holes, herein referred to as the remote end.
Once inserted, the screws are driven into the bone, therein securing the plate to the respective bone segments. The compressing function of the plate is utilized by continued driving of the screw into the plate once the annularly extending portion of the screw head has seated against the recessed lip of the hole. The downward force applied to the screw causes the head to slide down the slope of the recessed lip, thereby causing the bone in which it is inserted to move relative to the fixed plate. As the slopes of the respective holes are mutually directed toward the middle of the plate, the relative motion of the screws down the loped lips, and the corresponding motion of the bone segments into which they have been inserted, is together. The surgeon is thereby able to draw displaced bones together, and to hold the bones together internally during the required healing time.
It is understood that multiple elongate holes per side of the plate are an expedient for providing initial insertion sight choice to the surgeon, as full compression of one screw per side is the limit (further compression by a second screw per side would be prevented by the first). The additional holes may, however, be used to provide additional fixation points, effectively providing for the insertion of holding screws which help hold the plate to the bone.
Unfortunately, however, dynamic compression plates suffer from a failure mechanism which is endemic to all orthopedic implantation devices which incorporate bone screws. This failure mechanism is referred to as screw pull-out. Associated with screw pull-out are two complementary phenomena; the first being the failure of the screw-to-bone coupling, and the second being screw migration caused by repetitive cyclical loading. The failure of the screw-to-bone coupling is generally the result of loading, either in the form of a single sharp force which disrupts the sheath-like core of bone which surrounds the screw, or cyclical loading which weakens the bone sheath until it is too weak to hold the screw.
In most orthopedic implantation devices which are fixed via screws, the failure of the core of bone around a screw does not necessarily mean that the screw will pull out. If there are other screws which can hold he load for the period of time necessary for the bone coupling to reform, the fracture site may heal before the cyclical loading causes the screw to migrate. With respect to a dynamic compression plate, however, failure of the bone-to-screw interface is a particular problem as the constant lateral compression force which was applied via the head of the screw being driven down the slope of the lip is reversed. As the insertion holding strength of the screw is compromised the nearly uncontrollable tendency is for the screw to be lifted out of the hole as the bone segments slide back into displaced positions. This is an understandably critical problem as the failure of the bone-to-screw interface permits immediate pull-out, as well as potential catastrophic failure of the bone healing process.
While variations of the DCP concept, specifically those which include additional holding screws for anchoring the plate once compression has been applied, have reduced the dangers associated with bone screw pull-out in dynamic compression plates, the risks associated with this failure mechanism have not been eliminated. For example, the continued loading of the plate, even if fully secured, may still lead to the migration of the screw from the hole. In many areas of the body, the migration of a screw from an implantation device threatens collateral damage outside the obvious orthopedic implications, for example migrating screws may perforate adjacent vasculature or damage tissues.
An additional concern for surgeons who implant screw plate assemblies of all kinds is that a bone screw which is implanted perpendicular to the plate is particularly weak because the region of the bone which must fail for pull-out to occur is only as large as the outer diameter of the screw threads. It has been found that for pull-out to occur for a pair of screws which are angled inward, "toe nailed", or ones which diverge within the bone, and which are locked to the plate, the amount of bone which must fail increases substantially as compared to pairs of screws which are implanted in parallel along the axis that the loading force is applied. It has, therefore, been an object of those in the art to provide a screw plate assembly which permits the screws to be entered into the bones at angles other than 90 degrees.
With respect to dynamic compression plates, while the ability to drive additional holding screws in at angles other than 90 degrees is understandably valuable, the ability to drive the translating screw into the elongated hole at an angle other than 90 degrees (angled in the direction of compression) is especially important. The systems of the prior art which have provided for angulation of screws have generally been limited to single angles, via drill guides and offset surfaces for the angled heads to seat to. Surgeons ideally need to have freedom with respect to insertion angles so that the individual variations in fracture type, and bone anatomy may be accounted for in the fixation means. In addition, there are restrictions with respect to offset surfaces in that the screws associated therewith are not capable of dynamically compressing via travelling along an elongate hole; circular symmetry prevents proper seating in such a situation.
It is, therefore, a principal object of the present invention to provide a new and novel dynamic compression plate having a polyaxial coupling of the screw to the plate, and especially to the sloped lip of the elongated holes therein, whereby a single plate is compatible with a wide range of screw-in angles.
It is also an object of the present invention to provide a dynamic compression screw plate design having a locking compression screw and hole, whereby the screws which are inserted into the elongate holes are prevented from migrating out of the hole, despite potential bone-to-screw failure.
It is also an object of the present invention to provide a dynamic compression plate assembly which is more sturdy and more versatile than previous designs.
Further, it is an object of the present invention to provide a screw plate design which provides the surgeon with the greatest freedom to choose the most desirable angle to direct the bone screws.
It is also an object of the present invention to provide an orthopedic screw plate assembly which includes simple and reliable means for fixing the plate to the bone.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.